Free Interest Calculator
Calculate interest earned on a principal amount using either simple or compound interest. Free. No signup required.
- Principal$8,000.00 · 78%
- Interest Earned$2,210.25 · 22%
Of the $10,210.25 you end up with, $8,000.00 is what you put in — the rest, $2,210.25, is interest compounding on top of itself.
- Total Amount$10,210.25
About the Free Interest Calculator
The naive way to estimate interest is to multiply the rate by the years and call it done — and for simple interest, that's exactly right. But most real accounts don't work that way: banks, bonds and loans typically compound, meaning each period's interest gets added to the balance and then itself starts earning interest. Over a few years the gap between the two methods is small; over a decade or two it's often the difference between doubling your money and not.
This calculator runs both models side by side so you can see the actual dollar gap, not just the concept. It's the tool people reach for when comparing a savings account's advertised rate to what they'd naively expect, or when a homework problem or a real quote needs the interest isolated from the principal.
Everything runs in your browser — the principal you type in, whatever it represents, never leaves your device or gets logged anywhere.
How it’s calculated
Simple interest uses I = P × r × t: the rate applies only to the original principal, every year, for the life of the investment or loan — a flat, linear amount of interest each period.
Compound interest uses A = P × (1 + r)ᵗ: after each year's interest is added, next year's interest is calculated on the new, larger balance. That's why the compound total pulls further ahead of simple interest the longer the money sits — it's earning interest on interest, not just on the original amount.
Frequently asked questions
What's the actual difference between simple and compound interest?
Simple interest is always calculated on the original principal, so it grows by the same dollar amount every year. Compound interest is recalculated on the growing balance each period, so the dollar amount it adds increases over time even at the same rate.
Is compound interest always better for me as a saver?
Yes, when you're the one earning it — compounding grows your balance faster than simple interest at the same rate. It's the opposite when you're the one paying it, which is why compounding debt like credit card balances is worth avoiding.
Does compounding frequency (monthly vs. annually) matter?
It does — more frequent compounding periods produce a slightly higher total for the same annual rate, since interest starts earning its own interest sooner. This calculator compounds annually; a quote that compounds monthly or daily will land a bit higher than what's shown here for the same stated rate.
Why does my bank's advertised rate not match what I calculate by hand?
Banks usually advertise an APY (annual percentage yield), which already bakes in compounding frequency, rather than a bare interest rate. Multiplying a bare rate by years the way simple interest works will usually undershoot what a compounding account actually pays.
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